The invention relates to a faceted optical substrate, a method of fabricating a faceted optical substrate and a backlight display comprising the faceted optical substrate.
Backlight computer displays and other optical systems often use layers of prismatic structures stacked and arranged perpendicular to one another. The structures are typically sandwiched between other optical substrates known as diffusers. For example, in backlight displays, brightness enhancement substrates use prismatic structures to direct light along a viewing axis (i.e., an axis normal to the display, or “on axis”). This enhances brightness of light viewed by the user of the display to allow the system to use less power to create a desired level of on-axis illumination. Substrates for turning light can also be used in a wide range of other optical designs, such as for projection displays, traffic signals, and illuminated signs.
Current commercial brightness enhancement substrates are characterized by a sharp cut-off in brightness between about 40 and 50 degrees off-axis. At angles beyond this cut-off there are side-lobes in the angular brightness distribution. These side-lobes can result in a waste of energy. The side-lobes are also undesirable in security applications since they allow light to reach unintended viewers.
U.S. patent application Ser. No. 10/065,981, incorporated by reference herein in its entirety, discloses a structural shape for the surface of an optical substrate such that the brightness of diffuse light departing from the surface of the optical substrate at certain off axis angles is reduced at the expense of a small reduction in peak brightness measured near the viewing axis. The net result is an overall increase in useful illumination. Such an optical substrate comprises a surface characterized by a cross section of at least one prism having a curved sidewall or facet.
Sometimes, however, these substrates have manufacturing defects. Also, the displays are sometimes deficient in brightness as a result of optical coupling. There is a need to obscure manufacturing defects and to decrease the optical coupling in substrates of these optical devices.